Such a boiler includes, inter alia, the following elements shown diagrammatically in FIG. 1:
a hearth C for burning the fuel in a circulating fluidized bed of particles, the walls of the hearth comprising vertical pipes in which an emulsion (liquid phase and vapor phase) of water circulates, which emulsion is produced by evaporation of water fed into the bases of said pipes; PA1 a separator member S (in general a cyclone) which, at the top of the hearth, recovers the flue gas and the solid particles ejected from the top of the hearth and directs the flue gas and said particles to respective ones of two distinct ducts; PA1 a first heat exchanger ET1 often connected to the gas duct S1 of the separator member S, the first heat exchanger being, for example, an economizer, a vaporizer, a superheater or a resuperheater; and PA1 a dense fluidized bed ET2 fed with particles from the separator member S, i.e. that is connected firstly to the particles duct S2 of said separator member and secondly to the bottom of the hearth C (it is also possible to provide a dense fluidized bed ET2B that is fed with particles collected in the hearth C, i.e. connected firstly to the extraction duct of the hearth and secondly to the bottom of the hearth), the dense fluidized bed including a second heat exchanger whose inlet is connected to the outlet of the first heat exchanger ET1.
In general, the second heat exchanger is made up of a plurality of nests of tubes, each nest comprising a plurality of zigzag tubes arranged in parallel planes that are in general vertical. The coolant fluid, commonly steam, circulates in the nests, and the entire heat exchanger is integrated in a rectangular case.
Usually, the nests are disposed as follows: in succession starting from one wall of the case, a first nest optionally separated from the wall by a space without any zigzag tubes, then a second nest optionally separated from the first nest by a space without any zigzag tubes; the space between the first nest and the second nest may contain a physical separation such as a low wall or a partition; Any additional nests are arranged analogously, so that the last nest is optionally separated by a space without any zigzag tubes from that wall of the case which is opposite from the wall facing the first nest.
The feed duct for feeding the dense fluidized bed ET2 with particles is thus connected to a wall of the case or to the empty space between the first nest and said wall, the particles being returned to the hearth C via the opposite wall or via an outlet connected to the empty space between the last nest and the opposite wall.
The intense mixing of the particles tends to make the temperature inside the case uniform. However, since the particles leave the case colder than when they were fed into the case, the temperature is not the same everywhere. The tubes situated in the vicinity of the particles inlet receive more heat than those which are close to the outlet.
In addition, when the power of the boiler increases, the power and therefore the size of the heat exchangers must increase. Unfortunately, temperature dispersion is related directly to the size of the rectangular case. Thus, the temperature of the steam produced by the first nest disposed at the particles inlet is greater than the temperature of the last nest disposed at the outlet, which is not desirable.
Furthermore, nests that all withstand the maximum temperature of the case are avoided because that would be a costly solution. Therefore as many types of nest are provided as there are nests in a heat exchanger, each type of nest being dimensioned for an operating temperature which corresponds to its place in the case. Thus, the many advantages of having a single manufacturing line are lost.